Structural member for aeronautical construction with a variation of usage properties

ABSTRACT

This invention relates to a process for manufacturing an aluminium alloy part with structural hardening as well as to structural members including monolithic structural members and to products prepared from such structural members. A suitable process of the present invention involves annealing in a linear furnace with a controlled temperature profile comprising at least two zones or groups of zones Z 1 , Z 2 . The length parallel to the axis of the linear furnace of each of the at least two zones or groups of zones Z 1 and Z   2  is generally at least about one meter.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.60/555,304, filed Mar. 23, 2004, the content of which is incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to strain hardened products andstructural members, particularly for aeronautical construction, made ofa heat treatment aluminium alloy. In particular, the present inventionrelates to so-called long products, in other words products having alength that is significantly more than their width or thickness,typically with a length equal to at least twice their width, andtypically at least 5 meters long. These products may be, for example,rolled products (such as thin plates, medium plates, thick plates),extruded products (such as bars, sections, tubes or wires), and forgedproducts.

2. Description of Related Art

Very large aircraft have very particular construction problems. Forexample, the assembly of structural members becomes more and morecritical, firstly because of the cost factor (riveting is a veryexpensive process), and secondly because they generate discontinuitiesin the properties of assembled parts.

To minimise assemblies, structural members can be prepared by integralmachining in thick plates; different functions such as wing skin andwing stiffener can then be integrated into these single-piece(monolithic) structural members. At the same time, the dimensions ofmonolithic structural members can be increased. This introduces newmanufacturing problems for these parts made by rolling, extrusion,forging or casting, since it is more difficult to guarantee uniformproperties in very large parts.

The preparation of monolithic parts with a controlled variation ofproperties has also been mentioned, which in theory provides a means ofbetter adapting properties of parts to the manufacturer's needs. EP 0630 986 (Pechiney Rhenalu) describes a process for manufacturingaluminium alloy plates with structural hardening with a continuousvariation in usage properties, in which final annealing is done in afurnace with a special structure comprising a hot chamber and a coldchamber, connected by a heat pump. This process has been used to obtainsmall parts with a length of about one meter made of a 7010 alloy, oneend of which is in the T651 state, while the other end is in the T7451state, wherein the process uses an isochronous annealing treatment. Thisprocess has never been developed industrially, since it is difficult tocontrol compatibly with quality requirements necessary in theaeronautical construction field. These difficulties tend to increaseeven further as the size of the parts increases, knowing that theintegration of two or more functions into one single structural memberis especially interesting for very large pieces. Moreover, there is noreal need for small mechanical parts with a continuous variation ofusage properties. Another problem that arises with this process, forexample as described in EP 0 630 986, is that the optimum durations ofthe T651 and T7451 treatments are different. Another problem that arisesis that a 7010 product in the T7451 state is typically obtained by anannealing treatment with two plateaus, whereas the T651 state isobtained by an annealing treatment with a single plateau.

SUMMARY OF THE INVENTION

A problem addressed by the present invention was to develop a processfor manufacturing structural members, particularly for aeronauticalconstruction, with a variation of usage properties for the manufactureof very long parts, that is sufficiently controllable, stable andreproducible under strict quality assurance and statistical processcontrol conditions that are typically required by aeronautics.

An object of the present invention was the provision of a process formanufacturing an aluminium alloy part with structural hardening,comprising:

-   -   solution heat treating a semi-finished rolled, extruded and/or        forged product, followed by quenching,    -   optionally conducting controlled tension with permanent        elongation of at least 0.5%, and    -   annealing,    -   wherein at least a portion of the annealing is done in a furnace        with a controlled temperature profile comprising at least two        zones or groups of zones Z₁, Z₂ with initial temperatures        T_(1 and T) ₂ in which the temperature variation around the set        temperature for each of the temperatures T_(1 and T) ₂ does not        exceed about ±5° C. (preferably ±4° C. and even better ±3° C.)        within the length of the zones or groups of zones, and further        wherein the difference between the set values of the initial        temperatures T_(1 and T) ₂ are greater than or equal to about        5° C. (preferably from about 10° C. to about 80° C. and even        better from about 10° C. to about 50° C., and still better from        about 20° C. to about 40° C.). The zones or groups of zones can        optionally be separated by a zone or a group of zones Z_(1,2)        called a “transition group” within which the initial temperature        varies from T_(1 to T) ₂ and wherein the length parallel to the        axis of the furnace of each of at least two zones or groups of        zones Z_(1 and Z) ₂ is at least about one meter (and preferably        at least about two meters).

In further accordance with the present invention, there is provided amonolithic structural member comprising an aluminium alloy withstructural hardening having a length L greater than its width B and/orthickness E. The structural member is particularly adapted foraeronautical construction, and advantageously includes at least twosegments P₁ and P₂ located on a different length of the structuralmember that have mechanical properties (measured at mid-thickness)selected from the group consisting of:

-   -   a) P₁: K_(IC(L-T))≧38 MPa{square root}m and P₂: R_(m)(L)≧580 MPa        (and preferably ≧590 MPa and even better ≧600 MPa    -   b) P₁: K_(IC(L-T))≧40 MPa{square root}m and P₂: R_(m)(L)≧580 MPa        (and preferably ≧590 MPa)    -   c) P₁: K_(IC(L-T))≧41 MPa{square root}m and P₂: R_(m)(L)≧580 MPa        (and preferably ≧590 MPa)    -   d) P₁: K_(IC(L-T))≧42 MPa{square root}m and P₂: R_(m)(L)≧590 MPa    -   e) P₁: K_(IC(L-T))≧39 MPa{square root}m and P₂: R_(m)(L)≧580 MPa        and P₂: R_(m)(TL)≧550 MPa    -   f) P₁: K_(IC(L-T))≧39 MPa{square root}m and P₂: R_(m)(L)≧580 MPa        and P₂: R_(p0.2)(L)≧550 MPa    -   i) P₁: K_(IC(L-T))≧39 MPa{square root}m and P₁: R_(m)(L)≧530        MPa, and P₂: Rm(L)≧580 MPa    -   j) P₁: K_(IC(L-T))≧40 MPa{square root}m and P₁: R_(m)(L)≧540        MPa, and P₂: Rm(L)≧590 MPa    -   k) P1: K_(app(L-T)(CCT406))≧125 MPa{square root}m and P2:        R_(m)(L)≧590 MPa.

In yet further accordance with the present invention, there is providedan aircraft comprising at least one wing manufactured from a structuralmember according to this invention wherein segment P₁ is located closeto the fuselage and segment P₂ is located close to a geometric tip ofthe wing.

Additional objects, features and advantages of the invention will be setforth in the description which follows, and in part, will be obviousfrom the description, or may be learned by practice of the invention.The objects, features and advantages of the invention may be realizedand obtained by means of the instrumentalities and combinationparticularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate a presently preferred embodimentof the invention, and, together with the general description given aboveand the detailed description of a preferred embodiment given below,serve to explain principles of the invention.

FIG. 1 diagrammatically shows the variation of static mechanicalproperties (curve 1) for example tensile or compression strength, anddynamic properties (curve 2), for example tolerance to damage, withinthe length of a wing panel according to the invention.

FIG. 2 shows the mechanical strength of a 34-meter long structuralmember according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

a) Terminology

Unless mentioned otherwise, all indications about the chemicalcomposition of alloys are expressed in mass percentage by weight basedon the weight of the alloy. Consequently, in a mathematical expression,“0.4 Zn” means 0.4 times the content of zinc, expressed as a masspercentage; this is applicable with any necessary changes to otherchemical elements. The designation of alloys follows The AluminumAssociation rules, known to those skilled in the art. Metallurgicalstates are defined in European standard EN 515. The chemical compositionof normalized aluminium alloys is defined for example in standard EN573-3. Unless mentioned otherwise, static mechanical characteristics, inother words the ultimate strength R_(m), the yield stress R_(p02) andthe elongation at failure A, are determined by a tensile test accordingto standard EN 10002-1, the location at which these pieces are taken andtheir direction being defined in standard EN 485-1. The toughness K_(IC)was measured according to standard ASTM E 399. The R curve is determinedaccording to standard ASTM 561. The critical stress intensity factorK_(C), in other words the intensity factor that makes the crackunstable, is calculated starting from the R curve. The stress intensityfactor K_(CO) is also calculated by assigning the initial crack lengthto the critical load, at the beginning of the monotonous load. These twovalues are calculated for a test piece of the required shape. K_(app)denotes the K_(CO) corresponding to the test piece that was used to makethe R curve test. Resistance to exfoliation corrosion was determinedaccording to the EXCO test described in standard ASTM G34.

Definitions given in European standard EN 12258-1 are applicable unlessmentioned otherwise. The term “plate” is used in this patent for allthicknesses of rolled products.

The term “machining” includes any process for removal of material suchas turning, milling, drilling, trimming, electroerosion, grinding,polishing, and chemical milling.

The term “extruded product” also includes products that have been drawnafter extrusion, for example by cold drawing through an extrusion die.It also includes hard drawn products.

The term “structural member” refers to an element used in mechanicalconstruction for which the static and/or dynamic mechanical propertiesare particularly important for the performance and integrity of thestructure, and for which a structure calculation is generally specifiedor done. It is typically a mechanical part that could endanger thesafety of the said construction and its users, passengers or others, ifit fails. For an aircraft, these structural members include particularlyelements making up the fuselage (such as the fuselage skin, fuselagestiffeners or stringers, bulkheads, fuselage circumferential frames,wings (such as wing skins), stringers or stiffeners, ribs and spars andthe tail fin composed particularly of horizontal and verticalstabilisers, and floor beams, seat tracks and doors.

The term “monolithic structural member” refers to a structural membermade from a single piece of rolled, extruded, forged or castsemi—finished product, without assembly, such as riveting, welding,bonding with another part.

A problem existing in the prior art can addressed by employing a methodwherein the temperature in a furnace, the internal length of which isgreater than the length of the piece to be heat-treated, is keptapproximately constant in at least two zones of a furnace of at leastone meter long, while it is significantly different in at least oneother zone of at least one meter long. This type of temperature profilecan be obtained by subdividing the furnace lengthwise into severaltemperature zones.

The present inventive process is applicable to all long metallicproducts, in other words, products with one dimension (called thelength) being significantly longer than the other two dimensions (width,thickness). The length is the largest dimension of the product.Typically, within the context of this invention, the length is at leasttwice as large as the other two dimensions. In preferred embodiments,the length is five or even ten times as large as the other twodimensions. Length normally applies to the longitudinal manufacturingdirection (rolling or extrusion direction); but it may be different insome cases. Products according to the present invention may be rolledproducts (such as plates or thick plates), extruded products (such asbars, tubes or sections), and forged products; these products may be asmanufactured or as machined.

For the purposes of this description, the “segments with extremeproperties” of a product mean the segments with the greatest differencein properties. Depending on the chosen manufacturing methods, thesesegments may be located close to the “ends in the geometric sense” (or“geometric ends”) of the product, or they may be elsewhere. The presentinvention can also be used to make parts in which at least one of thetwo segments with the greatest difference in properties is closer to thegeometric center than to the geometric end of the part.

For the purposes of this description, a “zone” of a furnace is thesmallest thermal unit along the length of the furnace and characterizedby an approximately constant temperature, in other words by atemperature variation parallel to the axis of the furnace that is small(typically less than one third) compared with the temperature differencethat characterizes the variation of the furnace temperature over itstotal length. This type of furnace zone includes heating and controlmeans that keep the temperature at an approximately constant valuewithin the zone. That is, the temperature variation around the settemperature inside such a zone preferably does not exceed about ±5° C.,and more preferably does not exceed about ±4° C. In a preferredembodiment, this difference does not exceed about ±3° C. Certainproducts may require a temperature variation not exceeding about ±2° C.In the other directions of the furnace, the temperature should be asconstant as possible. In any case, the temperature variation around theset temperature within one zone should preferably be smaller than thevariation of temperature between the coolest and the hottest zone of thefurnace.

Several contiguous zones may form a “group of zones”, in other words athermal unit within which the temperature is approximately constant (asdefined above), forming a controlled temperature gradient. For example,a group of zones in a linear furnace could contain 9 furnace zones(numbered from 1 to 9), wherein two groups of temperature zones areformed, each comprising three furnace zones (numbered 1, 2, 3, 7, 8 and9 successively) separated by a central group of zones in which there isa controlled temperature gradient obtained using three furnace zones(numbered 4, 5 and 6 successively). For the purposes of the presentdescription, the term “zone group” may include only a single furnacezone.

According to observations made by the applicant, the minimum temperaturedifference that results in differences in properties that can be usedindustrially between two segments with extreme properties of a productaccording to the present invention, is preferably not less than aboutfive degrees. A difference of at least ten degrees is preferred in somecases. The temperature difference may be much greater, e.g. up to 80° C.or 100° C., or even more, but this can cause problems in control of thetemperature and the temperature profile parallel to the axis of thefurnace, particularly in the case of relatively small parts. Ifage-hardened tempers are to be obtained, the temperature differenceshould typically not exceed fifty degrees. A temperature difference ofmore than fifty degrees can advantageously be used to make a part forwhich one of the segments with extreme properties is in a temper closeto T3 or T4. For alloys of the Al—Zn—Cu—Mg type (series 7xxx), a rathersmall temperature difference (from about ten to about thirty degrees)leads to an effect which can be exploited, if desired, in structuralmembers for aircraft construction, while alloys of the Al—Cu type(series 2xxx) usually involve larger temperature differences, such as avalue from about 50 to about 100 degrees, or even higher.

The applicant has observed that it is not only the temperaturedifference between the two segments with extreme properties thatmatters, but also the temperature control of the temperature between thesegments with extreme properties. This is why the present inventionpreferably uses a furnace comprising a plurality of contiguous furnacezones. “A plurality” means at least two, and preferably at least threefurnace zones. A partition between two contiguous zones, as recommendedin EP 630 986, is not necessary or required. That is, the use of apartition may not enable sufficient control over the temperature betweentwo zones. Similarly, the use of a heat pump connecting the cold chamberto the hot chamber, as suggested in EP 630 986, may make the temperatureprofile inside the furnace too unstable. Within the context of thepresent invention, good control of the temperature profile within thefurnace is desirable in order to be able to manufacture structuralmembers compatibly with quality assurance requirements for aeronauticalproducts.

For this purpose, it is highly advantageous to be able to control, andpreferably regulate, the temperature in each furnace zone. In oneadvantageous embodiment of this invention, the furnace comprises atleast three furnace zones with a unit length of at least about onemeter. For example, to manufacture structural members with a length ofabout thirty-four meters, the inventors preferably use a furnace with atotal length of thirty-six meters with thirty furnace zones withapproximately equal lengths, preferably adjustable independently of eachother. Advantageously, these thirty furnace zones are grouped so as toform a small number of groups of temperature zones, for example three tofive groups.

A process according to the invention advantageously includes theproduction of a strain hardened part made of an aluminium alloy withstructural hardening, solution heat treatment, quenching, possiblytension with a permanent elongation of at least 0.5%, and an annealingtreatment in a furnace with a controlled temperature profile. Theannealing treatment in a furnace with a controlled temperature profilemay comprise one or several temperature plateaus, and typically two orthree, or a more or less continuous temperature ramp with no clearlydefined plateau, for at least one of the groups of temperature zonesmaking up the temperature profile. Optionally, the annealing treatmentin a furnace with a temperature profile is preceded or is followed byanother annealing treatment step in a homogeneous furnace (that may bethe same furnace, adjusted so as to obtain a uniform temperature in allzones, or another furnace). Such final annealing in a homogeneousfurnace is particularly useful when the objective is to obtain a temperwhich can be used for age forming. In this case, the final anneal isused for age forming. In another embodiment, a part may be annealed in afurnace with a controlled temperature profile, following by at least oneforming or machining operation, and then an annealing treatment step ina homogeneous furnace.

The invention can be used to make a monolithic structural member made ofan aluminium alloy with structural hardening with a length L greaterthan its width B and thickness E, particularly for aeronauticalconstruction, the monolithic structural member preferably wherein atleast two segments P₁ and P₂ on different lengths of the structuralmember have physical properties (measured at mid-thickness) selectedfrom the group formed of:

-   -   a) P₁: K_(IC(L-T))>38 MPa{square root}m and P₂: R_(m)(L)>580 MPa        (and preferably >590 MPa and even better >600 MPa    -   b) P₁: K_(IC(L-T))>40 MPa{square root}m and P₂: R_(m)(L)>580 MPa        (and preferably >590 MPa)    -   c) P₁: K_(IC(L-T))>41 MPa{square root}m and P₂: R_(m)(L)>580 MPa        (and preferably >590 MPa)    -   d) P₁: K_(IC(L-T))>42 MPa{square root}m and P₂: R_(m)(L)>590 MPa    -   e) P₁: K_(IC(L-T))>39 MPa{square root}m and P₂: R_(m)(L)>580 MPa        and P₂: R_(m)(TL)>550 MPa    -   f) P₁: K_(IC(L-T))>39 MPa{square root}m and P₂: R_(m)(L)>580 MPa        and P₂: R_(p0.2)(L)>550 MPa    -   i) P₁: K_(IC(L-T))>39 MPa{square root}m and P₁: R_(m)(L)>530        MPa, and P₂: Rm(L)>580 MPa    -   j) P₁: K_(IC(L-T))>40 MPa{square root}m and P₁: R_(m)(L)>540        MPa, and P₂: R_(m)(L)>590 MPa    -   k) P1: K_(app(L-T)(CCT406))>125 MPa{square root}m et P2:        R_(m)(L)>590 MPa.

It is preferable if the process is carried out such that the elongationat failure A(L) is greater than 9% and preferably >10% in segments P₁and P₂. This is advantageous particularly when the parts are to besubjected to forming operations after aging. Similarly, it is preferablethat A(L) is more than 9% outside these segments P₁ and P₂. It ispossible to manufacture semi-products in which (measured atmid-thickness)

-   -   a) R_(p0.2), determined in the L direction or in the LT        direction, has a difference p_(0.2(P2))-R_(p0.2(P1)) of at least        50 MPa and preferably of at least >75 MPa, and/or    -   b) R_(p0.2), determined in the ST direction, has a difference        R_(p0.2(P2))-R_(p0.2(P1)) of at least 30 MPa and preferably at        least 50 MPa, and/or    -   c) K_(IC), measured in the L-T direction, has a difference        K_(IC(P1))-K_(IC(P2)) of at least 5 MPa{square root}m and        preferably of at least 7 MPa{square root}m, and/or    -   d) K_(app), measured in the L-T direction, has a difference        K_(app(P1))-K_(app(P2)) of at least 10 MPa{square root}m and        preferably of at least 15 MPa{square root}m.

A process according to the invention may be used to producesemi-finished products made of any alloy with structural hardening, suchas aluminium alloys in the 2xxx, 4xxx, 6xxx and 7xxx series, and alloyswith structural hardening such as those in the 8xxx series containinglithium.

A process according to the invention may be used, in the case ofAl—Zn—Cu—Mg-type alloys (series 7xxx), for example, to put one of thesegments with extreme properties in a temper close to T6, and anothersegment with extreme properties in a temper close to T74 or T73.

In alloys of the 2xxx or 6xxx series, as well as in lithium-containingalloys of the 8xxx series, a process according to the invention may beused, for example, to put one of the segments with extreme properties ina temper close to T3 or T4, and the other segments with extremeproperties in a temper close to T6 or T8.

In one advantageous embodiment of the invention, the alloy comprisesfrom about 7 to about 15% of zinc, from about 1 to about 3% of copperand from about 1.5 to about 3.5% of magnesium. In other advantageousembodiments, the zinc content is at least about 7%, and preferably fromabout 8 to about 13%, and more preferably from about 8.5 to about 11%.The copper content is advantageously from about 1.3 to about 2.1%, andthe magnesium content is preferably from about 1.8 to about 2.7%. Thesealloys, including 7449, 7349 and 7056, can result in a very highmechanical strength (for example in the T651 or T7951 state) and veryhigh toughness (for example in the T76, T7651 or T74 state, or in theT7451, T73 or T7351 state) while keeping acceptable corrosion resistanceand compromise between mechanical strength and toughness, as well as anacceptable (i.e. at least EA rating) resistance to exfoliation corrosion(EXCO test) in the two states corresponding to two segments with extremeproperties of the product and in intermediate zones.

In one advantageous embodiment of this invention, annealing is carriedout on a plate, section or a forged part subjected to solution heattreatment, quenched and stretched, for example, in at least two steps:

A first homogenous step at a temperature between 115° C. and 125° C. fora duration of between 2 and 12 hours, and a second step during which onesegment or end is treated at a temperature between 115° C. and 125° C.,while the another segment or the other end is treated at a temperaturebetween 150° C. and 160° C., both for a duration of between 8 and 24hours.

This annealing is particularly suitable for products made of 7xxx alloy,and particularly 7349, 7449 or 7056 alloy.

In another advantageous embodiment of this invention, annealing is doneat about 120° C. (i.e. under-aging) on one segment or end P₁ of aproduct made of 2xxx alloy (such as 2024 or 2023), while annealing tothe peak mechanical strength (temper T851) at about 190° C. is carriedout on another segment or the other end P₂. In a variant of thisembodiment, the segment or end which is not peak-aged (i.e. P₁) is agedat about 100° C. (or 80° C.).

In another advantageous embodiment, annealing to the peak mechanicalstrength (temper T651) is carried out on a segment or end of productmade of a 7xxx alloy (such as 7349, 7449 or 7056) at about 120° C.,while over-annealing (temper T7651, T7451 or T7351) is carried out atanother segment or the other end in two plateaux at 120° C. and 150-165°C.

In yet another advantageous embodiment, annealing to the peak mechanicalstrength (state T6) is carried out on a product made of a 6xxx alloy(such as 6056) at about 190° C., while over annealing (state T7851) iscarried out in two plateaux at the other end.

Metallic parts obtained by the process according to the invention can beused as structural members in aeronautical construction. Thesestructural members may be bi-functional or multi-functional, in otherwords they may combine different functions in a single monolithic partthat processes in prior art could only combine by assembly of differentparts. These structural members of the present invention can also enablesimpler and lighter weight construction and manufacturing of aircraft,particularly very high capacity freight or passenger aircraft.

One specific advantage of the process according to the invention is thatoptimum properties are achieved at each segment with extreme propertiesor at each end, over a well-controlled length of the product. Thereforethe aircraft designer knows exactly the length over which the productwill have the recommended and guaranteed optimum properties. In oneparticularly preferred embodiment, a process according to the inventionis used to make structural members that do not have a continuousvariation of properties along their entire length, but in which thereare at least two zones in which the physical properties (or at leastsome of the physical properties) are constant over a certain length ofthe product. In one advantageous embodiment of the invention, the lengthof this zone is at least one meter, and preferably at least two meters.Such a product, as well as its use as a structural element in anaircraft wing, is schematically represented on FIG. 1.

Another specific advantage of the process according to the invention isprecise control of properties in the transition segment P_(1,2) betweentwo groups of segments P₁ and P₂ (there may be two or more groups,depending on the number of groups of temperature zones), wherein P₁ andP₂ may be segments with extreme properties. The aircraft designer doesnot need maximum properties in the transition zone for any particularproperty (or groups of properties) to be optimised, for example theultimate strength in the longitudinal direction R_(m(L)) and thetoughness K_(IC(L-T)). But he does need a certain compromise betweenthese properties or groups of properties, since in this transition zonethe structural member actually plays a structural role and must satisfyprecise specifications.

In particular, structural members include:

-   -   upper or lower wing (skin) panels;    -   upper or lower wing stringers;    -   wing spars;    -   fuselage stiffeners;    -   butt straps, particularly butt straps for upper and lower wing        (skin) panels;    -   fuselage panels.

The process according to the invention can be used for heat treatment oflong parts or structural members. Usually, their section perpendicularto the length is approximately constant over their length, but this isnot necessarily the case. Similarly, parts may or may not be straight;for example slightly curved forged structural members could be treated.The process could also be used to treat cast parts, but long cast partsare very unusual and difficult to make. In one preferred embodiment, thelength of the part is at least 5 meters, preferably at least 7 meters,but a length of 15 meters or at least 25 meters is preferable, to takefull advantage of the possibilities of creating several functionalisedsegments distributed over the length of the part. Thus, structuralmembers have been made with at least two zones P₁ and P₂ in which thelength F_(P1) and F_(P2) (expressed in percent of the total length L) ofthe said at least two segments P₁ and P₂ is such that F_(P1)>25% andF_(P2)>25% and preferably F_(P1)>30% and F_(P2)>30%. In otherembodiments, F_(P1)>35% and F_(P2)>30% or F_(P1)>40% and F_(P2)>30%.

Structural members according to the invention may advantageously be usedin aeronautical construction. For example, a high capacity aircraftincluding at least one wing including at least one structural memberaccording to the invention could be used, characterised in that segmentP₁ is located close to the fuselage, and segment P₂ is close to thegeometric tip of the wing (see FIG. 1). In one advantageous embodiment,the said wing (skin) panels are at least 15 meters long, and preferablyat least 25 meters long. As described in the example below, theinventors have made wing (skin) panels more than 30 meters long.

The parts and structural members of the present invention may bemonolithic. The process according to the invention can also be used forheat treatment of parts or structural members that are not monolithic,but are assembled from at least two rolled, extruded or forged parts orsemi-finished parts (preferably made from an aluminium alloy withstructural hardening), for example by welding, riveting or bonding. Itis also possible that one or several parts in such an assembly could bemade from a base material other than an aluminium alloy.

In this embodiment, it would, for example be possible to start by makingan assembly between at least one aluminium alloy plate with structuralhardening and at least one aluminium alloy section with structuralhardening by riveting, welding or bonding, the said assembly then beingtreated by the process according to the invention. In one advantageousembodiment of this variant of the process according to the invention,the plates and sections are in the T351 state, and the assembly is madeby laser beam welding (LBW), friction stir welding (FSW) or electronbeam welding (EBW). The applicant has observed that it may be preferableto treat such a welded assembly after welding by the process accordingto the invention, instead of treating the semi-finished products (platesand sections) that will be used in the said assembly before welding,since this can improve the mechanical strength of the welded joint andits resistance to corrosion. This effect is significant when the weldedjoint is spread over a long length of the structural member (for exampleapproximately parallel to the longitudinal direction of the product).

The invention will be better understood after reading the followingexample that is in no way limiting.

EXAMPLE

A 36-meter long, 2.5-meter wide and 30 mm thick plate is made by hotrolling of a rolling plate.

The alloy composition was:

-   -   Zn 9.1%, Mg 1.89%, Cu 1.57%, Fe 0.06%, Si 0.03%, Ti 0.03%, Zr        0.11%, other elements <0.01 each.

The rolling plate was homogenised for 14 hours at 475° C. The inputtemperature to the hot roller was 428° C., and the output temperature ofthe hot rolled plate was 401° C. The plate was solution heat treated,quenched and tensioned under the following conditions: holding for 6hours at 471° C., quenching in water at a temperature between about 15and 16° C., then controlled tension with a permanent elongation of about2.5%. The plate was then cropped to give a 34-meter long plate. It wasplaced lengthwise in a furnace composed of thirty 1200 mm long zones.All annealing temperatures were adjusted within an interval of less than±3° C. around the set value.

The annealing treatment consisted of a first homogenous treatment stepfor 6 hours at 120° C. (“first plateau”) and was immediately followed bya second step during which one 18-meter geometric tip (called Z₁,corresponding to 15 furnace zones) was treated for 15 hours at 155° C.(“second plateau” preceded by an adjustment period of about 1 hour),while the other 10.8-meter geometric tip (called Z₂, corresponding to 9furnace zones) was held for 16 hours at 120° C. The transition zonebetween these two tips was 7.2 meters long (called Z_(1,2) correspondingto 6 furnace zones).

After this second step, the electrical conductivity of the plate wasmeasured at different locations:

-   -   Segment P₁: between 18.2 and 19.5 MS/m    -   Segment P₂: between 22.5 and 23.5 MS/m    -   Segment P_(1,2): between 18.2 and 23.6 MS/m.

The plate was then subjected to a third annealing step, namelyhomogeneous annealing consisting of a temperature increase to 148° C.for 1h30, followed by holding at 150° C. for 15 hours. This third stepwas intended to simulate age forming or annealing after the structuralmember was shaped.

The plate was cut and characterised. Table 1 summarises the staticmechanical properties obtained by a tension test. These are averagesobtained from measurements made at mid-thickness and at differentlocations distributed along the plate width. No significant variation ofproperties was observed in the plate width. For R_(P0.2) in the L and LTdirection, values have also been obtained by compression; these valuesare put between brackets in table 1. TABLE 1 LT TC Position [mm] L(long) (long transverse) (short transverse) in the length directiondirection direction of a 34 m R_(m) R_(p0.2) A R_(m) R_(p0.2) A R_(m)R_(p0.2) A panel [MPa] [MPa] [%] [MPa] [MPa] [%] [MPa] [MPa] [%]   0(P₁) 561 517 13.5 550 506 12.5 550 495 8.5 (509) (519) 13600 (P₁) 565522 13.5 553 511 12.5 548 502 8.5 (513) (528) 16000 (P₁) 556 509 13.5547 501 12.5 540 500 8.5 (500) (514) 18400 (P_(1,2)) 566 523 13.5 559519 12.5 546 498 7.5 (527) (538) 20800 (P_(1,2)) 612 587 12.0 598 57511.5 590 545 7.0 (573) (593) 25600 (P₂) 621 598 12.5 607 585 11.5 595554 6.5 (590) (605) 34000 (P₂) 624 602 12.1 608 586 11.5 599 558 6.1(594) (607)

The toughness results K_(IC) and K_(app) (the latter obtained on a CT127type test piece as well as on a CCT406 type test piece) are given intable 2 TABLE 2 Position [mm] along K_(app)(L − T) K_(app)(L − T) thelength of a K_(IC) (L − T) K_(IC) (T − L) (CT127) (CCT406) 34 m panel[MPa{square root}m] [MPa{square root}m] [MPa{square root}m] [MPa{squareroot}m]   0 (P₁) 43.8 36.1 106 132 13600 (P₁) 45.8 38.1 108 — 16000 (P₁)46.7 37.3 99 — 18400 (P_(1,2)) 43.0 34.2 102 — 20800 (P_(1,2)) 39.4 32.988 — 25600 (P₂) 36.1 34.9 89 34000 (P₂) 34.9 29.1 94 110

This 34-meter long plate can be used as a wing (skin) panel for veryhigh capacity cargo or passenger aircraft. For this application, thesegment with extreme properties X of the plate (corresponding to a hightoughness K_(IC), the static mechanical strength being lower) is fittedon the fuselage side and the segment with extreme properties Z of theplate (corresponding to a high static mechanical strength with a lowertoughness K_(IC)) is at the geometric tip of the wing.

The temperature set points as well as the temperature measure on theplate and in the air of the furnace zones during the second aging stepare shown in table 3. It includes the temperature profile during theannealing step at 120° C. and 155° C. at a steady temperature state. Thetemperature of the plate was measured using about forty thermocouples;the values given in table 3 were measured at mid-width. TABLE 3 FurnaceSet Plate Air temperature zone temperature [° C.] temperature [° C.] [°C.] 1 120 3 120 120.5 6 120 120.8 120.8 9 120 124.4 124.3 10 123 125.9126.7 11 129 129.9 129.7 14 147 147.7 148.3 16 155 157.2 156.6 17 155156.8 156.6 18 155 155.3 154.9 22 155 155.1 154.8 30 155

Additional advantages, features and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details, and representativedevices, shown and described herein. Accordingly, various modificationsmay be made without departing from the spirit or scope of the generalinventive concept as defined by the appended claims and theirequivalents.

As used herein and in the following claims, articles such as “the”, “a”and “an” can connote the singular or plural.

All documents referred to herein are specifically incorporated herein byreference in their entireties.

1. A process for manufacturing an aluminium alloy part with structuralhardening, comprising: solution heat treating a semi-finished rolled,extruded or forged product, followed by quenching, optionallycontrolling tension with permanent elongation of at least 0.5%, andannealing, wherein at least a portion of the annealing is conducted in afurnace with a controlled temperature profile comprising at least twozones or groups of zones Z₁, Z₂ with initial temperatures T_(1 and T) ₂and having a temperature variation around the set temperature for eachof the temperatures T_(1 and T) ₂ that does not exceed about ±5° C.within the length of the zones or groups of zones, and wherein thedifference between the set values of the initial temperaturesT_(1 and T) ₂ is greater than or equal to about 5° C., and the zones orgroups of zones are optionally separated by a zone or a group of zonesZ_(1,2), within which the initial temperature varies from T_(1 to T) ₂,and wherein the length parallel to the axis of the furnace of each ofthe at least two zones or groups of zones Z₁ and Z₂ is at least onemeter.
 2. A process according to claim 1, wherein the temperaturevariation around the set temperature for each of the temperatures T₁ andT₂ does not exceed about ±4° C. within the length of the at least twozones or groups of zones Z₁ and Z₂.
 3. A process according to claim 1,wherein the difference between the set temperatures T₁ and T₂ is fromabout 10° C. to about 80° C.
 4. A process according to claim 1, whereinthe temperature in at least one of the zones or groups of zones Z₁ or Z₂varies as a function of time according to at least two temperatureplateaus, and/or according to a temperature ramp with no clearly definedplateau.
 5. A process according to claim 1, wherein the annealing in alinear furnace with controlled temperature gradient is followed by atleast one forming or machining operation and annealing in a homogeneousfurnace.
 6. A process according to claim 1, wherein the annealing in alinear furnace with a controlled temperature gradient is preceded byannealing in a homogeneous furnace.
 7. A process according to claim 1,wherein the length of the part is at least 7 meters.
 8. A processaccording to claim 1, wherein the aluminium alloy part with structuralhardening is monolithic.
 9. A process according to claim 1, wherein thealuminium alloy part with structural hardening is assembled startingfrom at least two aluminium alloy parts with structural hardening.
 10. Aprocess according to claim 9, wherein assembly of said at least twoparts are made by riveting, bonding, laser beam welding, friction stirwelding and/or electron beam welding.
 11. A process according to claim1, wherein the annealing comprises a first homogeneous treatment at atemperature between 115° C. and 125° C. for a duration of from about 2to about 12 hours, a second treatment during which one end of said partis treated at a temperature from about 115° C. to about 125° C., whilethe other end of said part is treated at a temperature from about 150°C. to about 160° C., both for a duration of between 8 and 24 hours. 12.A monolithic structural member comprising an aluminium alloy withstructural hardening having a length L greater than a width B andthickness E, suitable for aeronautical construction, said monolithicstructural member comprising at least two segments P₁ and P₂ eachlocated on a different length of said structural member, wherein atleast one physical property (measured at mid-thickness) of P₁ and/or P₂selected from the group consisting of: a) P₁: K_(IC(L-T))≧38 MPa{squareroot}m and P₂: R_(m)(L)≧580 MPa b) P₁: K_(IC(L-T))≧40 MPa{square root}mand P₂: R_(m)(L)≧580 MPa c) P₁: K_(IC(L-T))≧41 MPa{square root}m and P₂:R_(m)(L)≧580 MPa d) P₁: K_(IC(L-T))≧42 MPa{square root}m and P₂:R_(m)(L)≧590 MPa e) P₁: K_(IC(L-T))≧39 MPa{square root}m and P₂:R_(m)(L)≧580 MPa and P₂: R_(m)(TL)≧550 MPa f) P₁: K_(IC(L-T))≧39MPa{square root}m and P₂: R_(m)(L)≧580 MPa and P₂: R_(p0.2)(L)≧550 MPai) P₁: K_(IC(L-T))≧39 MPa{square root}m and P₁: R_(m)(L)≧530 MPa, andP₂: Rm(L)≧580 MPa j) P₁: K_(IC(L-T))≧40 MPa{square root}m and P₁:R_(m)(L)≧540 MPa, and P₂: Rm(L)≧590 MPa k) P₁: K_(app(L-T)(CCT406))>125MPa{square root}m et P2: R_(m)(L)>590 MPa.
 13. A structural memberaccording to claim 12, wherein A_((L))≧9% in segments P₁ and P₂.
 14. Astructural member according to claim 13, wherein A_((L))≧9% outsidesegments P₁ and P₂.
 15. A structural member according to claim 12,wherein the length F_(P1) and F_(P2) (expressed as a percent of thelength L) of said at least two segments P₁ and P₂ is such thatF_(P1)≧25% and F_(P2)>25%.
 16. A structural member according to claim15, wherein F_(P1)≧35% and F_(P2)≧30%.
 17. A structural member accordingto claim 16, wherein F_(P1)≧40% and F_(P2)≧30%.
 18. A structural memberaccording to claim 12, wherein the alloy comprises from about 7 to about15% of zinc, from about 1 to about 3% of copper and/or from about 1.5 toabout 3.5% of magnesium.
 19. A structural member according to claim 18,wherein zinc is from about 8 to about 13%.
 20. A structural memberaccording to claim 19, wherein copper is from about 1.3 to about 2.1%.21. A structural member according to claim 20, wherein magnesium is fromabout 1.8 to about 2.7%.
 22. A structural member according to claim 12,wherein the length of the part is at least 7 meters.
 23. A method formaking an aircraft wing panel, wing stringers, wing spars, fuselagestiffeners, fuselage panels and/or butt straps comprising using astructural member according to claim
 12. 24. An aircraft comprising atleast one wing panel made from a structural member according to claim12, wherein said segment P₁ is located close to the fuselage, and saidsegment P₂ is close to the geometric tip of the wing.
 25. A method forforming a hardened aluminium alloy part comprising treating said part ina furnace having at least two zones, each at least one meter in lengthat a temperature that is maintained approximately constant in said atleast two zones.
 26. A monolithic structural member prepared from aprocess of claim
 25. 27. An aircraft comprising a structural member ofclaim
 26. 28. A semi-product in which (measured at mid-thickness)comprising an aluminium alloy with structural hardening having a lengthL greater than a width B and thickness E, suitable for aeronauticalconstruction, said semi-product comprising at least two segments P₁ andP₂ each located on a different length of said semi-product, wherein atleast one physical property (measured at mid-thickness) of P₁ and/or P₂selected from the group consisting of: a) R_(p0.2), determined in the Ldirection or in the LT direction, has a difference_(p0.2(P2))-R_(p0.2(P1)) of at least 50 MPa and preferably of atleast >75 MPa, and/or b) R_(p0.2), determined in the ST direction, has adifference R_(p0.2(P2))-R_(p0.2(P1)) of at least 30 MPa and preferablyat least 50 MPa, and/or c) K_(IC), measured in the L-T direction, has adifference K_(IC(P1))-K_(IC(P2)) of at least 5 MPa{square root}m andpreferably of at least 7 MPa{square root}m, and/or d) K_(app), measuredin the L-T direction, has a difference K_(app(P1))-K_(app(P2)) of atleast 10 MPa{square root}m and preferably of at least 15 MPa{squareroot}m.
 29. A single monolithic structural member that is at leastbi-functional.
 30. A structural member of claim 29 comprising an alloyselected from the group consisting of 7449, 7349 and
 7056. 31. Asemi-product of claim 28, wherein said alloy is selected from the groupconsisting of 7449, 7349 and
 7056. 32. A structural member of claim 29that does not have a continuous variation of properties along its entirelength, and said structural member comprises at least two segments inwhich at least some physical properties thereof are constant over apredetermined length of the segment.
 33. A member of claim 32, whereinsaid predetermined length is at least one meter.
 34. A member of claim32, wherein said predetermined length is at least two meters.
 35. Amethod of claim 25 wherein the product produced thereby does not have acontinuous variation of properties along its entire length, and saidproduct produced comprises at least two segments in which at least somephysical properties thereof are constant over a predetermined length ofthe segment.
 36. A method of claim 35, wherein said predetermined lengthis at least one meter.
 37. A method of claim 35, wherein saidpredetermined length is at least two meters.
 38. A structural member ofclaim 26 that does not have a continuous variation of properties alongits entire length, and said structural member comprises at least twosegments in which at least some physical properties thereof are constantover a predetermined length of the segment.
 39. A member of claim 38,wherein said predetermined length is at least one meter.
 40. A member ofclaim 38, wherein said predetermined length is at least two meters. 41.An aircraft comprising a structural member of claim
 38. 42. An aircraftcomprising a structural member of claim
 39. 43. An aircraft comprising astructural member of claim
 40. 44. A process of claim 1, wherein saidaluminum alloy is selected from the group consisting of 2xxx, 4xxx,6xxx, 7xxx and 8xxx alloys.
 45. A structural member of claim 12 whereinsaid aluminum alloy is selected from the group consisting of 2xxx, 4xxx,6xxx, 7xxx and 8xxx alloys.
 46. A method of claim 25 wherein saidaluminum alloy part comprises 2xxx, 4xxx, 6xxx, 7xxx and/or 8xxx alloys.47. A monolithic structural member prepared using a method of claim 46.48. An aircraft comprising a monolithic structural member of claim 47.